Low-density porous products offer unique mechanical and physical properties. The high specific strength, structural rigidity and insulating properties of foamed products produced in a polymer type matrix are well known. Such closed cell polymeric foams are used extensively in a wide range of applications, including construction, packaging and transportation.
While polymeric type foams have enjoyed wide market success, foamed metal products have seen only limited applications. Closed cell metallic foams offer many of the attractive attributes of polymeric foam with respect to many light weight applications. In addition, the inherently higher bulk modulus of metals, as compared to polymers, provides higher specific rigidity. This higher bulk modulus makes metal foams an attractive candidate for core materials in laminate panels, in which rigidity and resistance to deflection are important performance measures. Additionally, panels produced from foamed metal are fire and smoke resistant and are well suited for construction applications. Aluminum foam core sandwich composite products offer the additional environmental benefit of being recyclable; an issue that has restricted the use of metal clad polymer foams.
While methods of producing foamed metals have been described in the scientific and patent literature, such materials suffer from problems such as high cost, large cell sizes, cell size variability and insufficient structural integrity. Many of these problems are associated with the rheology of the molten metal. In substantially all casting metallurgy methods of producing metallic foams some stabilization is required in the metallic melt. Foams are meta-stable and therefore prone to both cellular coalescence and cell wall drainage.
Conventionally, in order to achieve the required stabilization for producing metal foams, particulates, such as ceramic particles, are introduced into the melt. These particulates effectively change the nature of the melt by increasing the effective viscosity of the melt and/or decreasing the effective surface tension of the liquid. These particulates must be small relative to the desired cell wall thickness of the foam. Incorporating small particulates into the melt is traditionally achieved using either intrinsic or extrinsic methods, wherein each method has disadvantages limiting their usefulness.
In intrinsic particle formation a gas is stirred into the molten metal, either by vortexing mechancal mixers and/or bubbling of gas (direct gas injection) through the melt. The gas reacts with the melt to form small particles including oxides, spinels and/or other unique particles. Controlling the size, geometry and volume fraction of the particles formed to create a stable, foamable matrix is particularly difficult.
The size of the particles formed is affected by the size of the gas bubbles injected or entrained. Producing small gas bubbles in liquid metal is notoriously difficult. Additionally, melt temperature, time at melt temperature, gas composition, stirring rate and melt composition all affect the rate, amount and characteristics of the particles and their distribution. Further, in aluminum melts, it is often necessary to add highly reactive alkali metals to promote such oxidation reactions.
One disadvantage of direct gas injection and/or stirring in providing foamed metals is the time required to create a stable foamable matrix. Time scales on the order of 20 minutes to several hours are often required even for small quantities of molten metal and larger quantities of molten metal often require much longer times to achieve the rheological character required to create a stable metal foam.
Extrinsic particle addition also suffers from a number of disadvantages which limit its usefulness as a method of stabilizing metal for foaming. In extrinsic particle addition small, inert particles are directly added and mixed into the melt. One disadvantage of extrinsic particle addition is that the extrinsically added particulates must be wetted so they remain suspended in the melt.
In an effort to wet extrinsically added particles, prior methods have utilized special alloying of the melt and/or particle coatings; sequencing of the melt alloy concentration and/or particulate addition; tight requirements on particle quality/surface composition; and elaborate equipment to control and enhance the wetting process between the particulates and the molten metal by imposing high shear in a vacuum or inert environment. These technical challenges translate into exotic processing equipment and limitations on the size and purity of the extrinsic particles used. These barriers have prevented the economical production of metal foams produced through extrinsically stabilized melts.
U.S. Pat. No. 3,297,431 to Ridgeway Jr. (“Ridgeway Jr.”) requires the use of stabilizer powders to maintain and preserve the cellular structure of aluminum foam upon cooling. As described in Ridgeway Jr., such stabilizing particles are finely divided inert powders which are wetted by the molten metal and are stable in the molten metal. The use of stabilizer particles is also described in U.S. Pat. No. 5,112,697 to Jin et al. (“Jin et al.”), in which Jin et al. defines precise limits on the size and volume fractions of such “finely divided stabilizer particles”. Additionally, U.S. Patent Application Publications U.S. 2004/0163492A1 and 2004/0079198A1 (Crowley et al. and Bryant et al. respectively) disclose the use of surface coatings on such viscosity control agents in foaming aluminum. All of these disclosures have their own disadvantages.
In light of the above-described obstacles and disadvantages, there is a need to provide a more commercially attractive means of metal foam production.